This invention relates in general to LED control circuits, and more particularly to a circuit for efficiently controlling current through an LED backlight panel.
Liquid Crystal Displays (LCDs) often incorporate backlight panels for to permit viewing in poor lighting conditions. The use of LEDs (Light Emitting Diodes) is known for the purpose of illuminating such LCD displays. Prior art LED backlight panels have used a plurality of parallel LED banks disposed behind the LCD display (e.g. four parallel banks of two series-connected LEDs per bank). In order to properly control the amount of illumination it is necessary to control the current passing through the LEDs. Since each of the LEDs is characterized by a voltage drop of from 2.0 V to 2.5 V, it is not possible to provide a controlled current supply from a standard 5 V supply voltage rail.
Thus, according to one prior art approach a linear voltage regulator has been configured to function as a current source for the LEDs (provided that there is sufficient voltage to drive the regulator). One disadvantage of this approach is that excessive power dissipation in the regulator results in high inefficiencies, particularly if the available voltage is unregulated and varies over a wide range. Furthermore, the regulator may be required to be fitted with a heat sink in order to limit the regulator temperature rise.
Another prior art approach overcomes the efficiency problems of the abovediscussed approach by doubling the standard regulated 5 V supply voltage though a charge pump (e.g. switched capacitor) or other voltage step-up circuit, in order to deliver approximately 9 V and using a low dropout voltage regulator configured as a current sink for controlling the LED current. The use of a regulated source voltage improves on efficiency relative to the first mentioned prior art approach, but considerable power must still be dissipated in the current sink circuit. This latter prior art approach is also costly and consumes valuable printed circuit board area.
According to the present invention, the LED backlight network is placed in the feedback path of an efficient switching power supply. The switching power supply directly controls the current through the LEDs and adjusts its output voltage as required to accommodate the forward voltage drop across the LEDs. Efficiency is much improved relative to the prior art since the power supply is able to provide the LEDs with only enough voltage to satisfy the current required for a desired brightness. A variation of the switching power supply circuit according to the invention further improves efficiency and limits the output voltage, but at the expense of an increase in LED current variation.